1. Field of the Invention
The present invention is directed to a method for processing an electrical signal, particularly a signal derived from a heart, of the type wherein the electrical signal is converted by delta modulation into a binary signal composed of a pulse sequence having time-discrete, logic operational sign values of zero or one.
2. Description of the Prior Art
A method of this general type is disclosed, for example, by European Application 0 402 508 or U.S. Pat. No. 4,567,883, wherein an electrical signal derived from a heart is compared in a delta modulator to a signal referred to as a predictor signal, that has either a constant positive or a constant negative slope, and a binary operational sign signal having operational sign values of one and zero is generated at the output of the delta modulator dependent on whether the electrical signal upwardly or downwardly exceeds the predictor signal. A change of the predictor signal from the positive slope to the negative slope occurs given every operational sign change from one to zero, whereas the operation sign signal is modified in the opposite way given an operational sign change from zero to one. The binary operational sign signal at the output of the delta modulator thus indicates to what extent the electrical signal follows the predictor signal with the two different, constant slopes. In order to obtain a better matching of the predictor signal to the curve of the electrical signal, and thus to keep the modulation noise, i.e. the error differences between the electrical signal and the predictor signal, optimally low, it is additionally provided in the method disclosed by U.S. Pat. No. 4,567,883 that different amounts can be set for the slope of the predictor signal dependent on the operational sign signal at the output of the delta modulator.
It is often desirable for a further evaluation of the delta-modulated signal to first store the pulse sequence of the binary operational sign signal in a memory and to recall it as needed from the memory for the signal evaluation. An example of this is the storing of an intracardial electrogram digitized by delta modulation in a memory of an implantable heart pacemaker or defibrillator in order to be able to utilize the stored, intracardial electrogram for diagnostic or therapeutic purposes at a later time. To make a meaningful evaluation of the intracardial electrogram possible, the electrogram must usually be acquired over a longer time span and be stored, so that an extremely large data set to be stored arises. The memory capacity that can be realized in implantable devices, however, is limited due to the small structural size and the compact format of such devices.